The goal of the proposed study is to advance the development of in situ remediation tools for mercury. There are few available remediation options for Hg contaminated sediments and soils, short of capping and dredging. In situ remediation has not been widely used for mercury, but offers significant advantages in cost and in preservation of ecosystem services especially for large areas with low/moderate levels of contamination. Two recent major advances improve the chances for development of effective in situ remediation options for Hg. First, recent lab and field trials show that in situ activated caron amendments, applied as a thin layer to undisturbed sediments or soils, can significantly reduce MeHg exposure from contaminated sites. Second, the identification of the gene pair responsible for microbial Hg methylation earlier this year (hgcAB) will allow the distribution and activity of Hg- methylating microorganisms in the environment to be established for the first time. Organisms that contain these genes appear to be relatively rare, and their distribution in nature remains unknown. This has the potential to significantly improve models of MeHg production. This research study will take advantage of both of these developments, and several other novel tools, to develop a better understanding of the controls on MeHg production and bioavailability, to develop in situ remediation approaches, and to identify biogeochemical characteristics that may make sites appropriate for these technologies. Using this new information, our goal is to develop an empirical model of the factors influencing Hg and MeHg bioavailability in contaminated areas to: 1) identify characteristics that make Hg- contaminated sites suitable for in situ sorbent remediation, and 2) design sorbent amendment/thin capping strategies that reduce MeHg bioavailability. The main study site will be a salt marsh in Berry's Creek, NJ, where we have just begun a field trial of in situ sorbent remediation using activated carbon. Additional field trials in the Berry's Creek marsh will be done to evaluate the relative efficacy o a wider range of black carbons. This intensive work in Berry's Creek will be supplemented with laboratory microcosm studies using sediments and soils from other Hg-contaminated sites. This approach will be used to evaluate the efficacy of a variety of black carbons across a wider range of biogeochemical and site conditions.